1. Field of the Invention
The invention relates to a process for producing sintered parts with high wear resistance and, simultaneously, good dynamic strength properties from pressed formed bodies, as well as to a device for implementing this process.
2. Description of the Prior Art
Steel parts subjected to great mechanical stress, such as toothed gearwheels, not only must have high dimensional accuracy, but also must possess very good dynamic strength properties and high wear resistance. For a long time, it seemed that the only feasible way to manufacture parts of this type was by machining processes followed by case hardening. However, in order to reduce forming expense, it is also possible to use powder-metallurgical processes. In this context, it is known to form pressed bodies in the form of green bodies from a diffusion-alloyed oil-hardened steel powder to which, along with standard lubricants, graphite has been added in a quantity corresponding to the desired C content. The green bodies are then sintered in a continuous process in a furnace and subsequently cooled to room temperature. In order to improve dimensional accuracy, another pressing step is subsequently carried out on a calibrating press. After this, case hardening with quenching in oil is carried out, followed by a tempering treatment. The parts produced in this manner display a typical tempered structure.
A manufacturing process of this sort produces parts with good static properties (tensile strength, hardness, wear resistance) as well as good dynamic strength properties. However, despite the expense, which results from the second pressing step (calibration), the dimensional accuracy and the uniformity thereof occasionally leave something to be desired. The attainable tolerance class is approximately IT10.
Furthermore, it is known to produce sintered parts from pressed bodies that have been pressed from completely-alloyed, air-hardened steel powder. When this is done, a martensitic structure is produced by cooling in air to below the martensitic start temperature. Although sintered parts of this type, because of their great hardness, have good wear properties, they are unsuitable for dynamic types of stress, such as those regularly experienced by toothed gears, due to their ductile yield. Furthermore, sintered parts produced in this manner are often unsatisfactory in respect to the attainable dimensional accuracy (tolerance class IT9).
Finally, it is known from DE 40 01 899 C1 that, in order to produce high-strength sintered parts, green parts are pressed from completely-alloyed steel powder with an added mass share of 0.3 to 0.7% carbon in the form of graphite powder, sintered at a temperature in a range from 1120.degree. to 1280.degree. C., hardened by cooling and then tempered.